A. Field of Invention
This invention pertains to a system for performing an electrophysiological study of a patient's cardiovascular system using an implantable cardiac device coupled to an external programmer. More particularly, the present invention pertains to a system which incorporates a capture detection feature to insure that stimulation pulses delivered to the heart for a particular test have the desired effect and hence the test is not wasted.
B. Description of the Prior Art
Frequently, a clinician must perform electrophysiological studies on a patient to determine accurately the patient's pathological condition, cardiovascular characteristics and other information he may need in order to prescribe a particular therapeutic treatment for the patient. Originally, these studies were invasive because they required the surgical insertion of probes and instruments into the patient. Thus, these studies were accompanied by some risk and preferably were performed in hospitals. Therefore these studies were not only expensive but also time consuming and caused patient anxiety.
In order to avoid medical risks to the patient and hospitalization, non-invasive programmed stimulation procedures have been developed (hereinafter referred to as NIPS) which could be used to perform the required studies during a routine office visit.
Moreover, as shall be seen below, these procedures require only an implantable cardiac rhythm management device and a programmer. The implantable rhythm management device may be a pacemaker, an implantable cardio-version device or an implantable defibrillator. For the sake of brevity the device shall be referred to as an ICD.
Typical NIPS procedures consist generally of the introduction of artificial premature electrical pulses at precise intervals to the myocardium. The cardiac response to these pulses can be used for example to evaluate cardiac tachyarrhythmias, assess the functioning of the SA node, the A-V node and the HIS Purkinje system, assess the efficacy of antiarrhythmic drugs, or the ICD itself, and so on.
Basically, a typical NIPS procedure consists of three steps. First, the cardiovascular system is stimulated by the application of stimulation pulses. These pulses may be applied either externally or internally. External pulses are preferably applied to the chest wall using standard external ECG electrodes positioned on the chest of the patient. For internal stimulation, the programmer takes over the operation of the ICD and uses its pulse generator to generate the required pulses. The purpose of the procedure is to either induce or to terminate an abnormal tachyarrhythmia. The stimulator pulses may be repeated.
Prior to the start of NIPS, a train of pulses is applied to the patient's heart. This train is normally referred to as the drive train and consists typically of eight pulses spaced about 300-400 msec. This train is typically followed by 1-4 extra pulses. The interval between the drive train and the first of these extra pulses is reduced gradually until it falls in the cardiac refractory period. The interval is then increased by 10 msec.
The next step in the typical NIPS procedure is to evaluate the response of the cardiovascular system to the stimulation by recording and displaying the response. The response may be detected using either internal sensing, external sensing or both. This evaluation may occur automatically, or may be performed by the clinician himself.
The third step of the procedure is to terminate the test.
However, a problem with the existing procedures is that typically the stimulation pulses are used to overdrive the heart and to impose on it a new rhythm. If the stimulation pulses within short intervals are applied during an absolute refractory period, then these stimulations are ineffective. However, it is difficult to ascertain the occurrence of such an event until the results of the simulation pulses are evaluated. Accordingly, every stimulation train and its results must be carefully reviewed by the clinician to insure that the heart has been properly stimulated. Moreover, if a particular procedure involving stimulation results in an unstimulated heart, the test is wasted and must be repeated.